Aerogels are solid objects derived from wet-gels by converting their pore-filling solvent into a supercritical fluid that is vented off like a gas. In principle, that process preserves the volume of the original wet-gel into the final dry object; thereby aerogels are highly porous, low-density materials. Conversely, simple evaporation of the pore-filling solvent causes extensive shrinkage, resulting in materials that are referred to as xerogels, which consist of the same elementary building blocks as aerogels. However, due to shrinkage-induced compaction, xerogels have lower porosities and higher densities than aerogels (e.g., see: Brinker, C. J., et al., Sol-Gel Science. The Physics and Chemistry of Sol-gel Processing. Academic Press: New York, 1990).
Silica is the most common type of aerogels, but a wide array of other inorganic and polymeric aerogels is known, including organic/inorganic interpenetrating networks (e.g., see: Leventis, N., Interpenetrating Organic/Inorganic Networks of Resorcinol-Formaldehyde/Metal Oxide Aerogels in Aerogels Handbook—Advances in Sol-Gel Derived Materials and Technologies. Aegerter, M.; Leventis, N.; Koebel, M. Eds., Springer: New York, N.Y., 2011, Chapter 14, pp 287-313), and polymer-crosslinked oxide aerogel composites (e.g., see: Leventis, N., Acc. Chem. Res., 40:874-884 (2007); While, L. S., et al., Transl. Mater. Res., 3:015002 (2006); Maleki, H., et al., J. Phys. Chem. C, 119:7689-7703 (2015); Mohite, D. P., et al., Chem. Mater., 24:3434-3448 (2012)). In the latter variety, the skeletal inorganic-oxide framework is coated conformally with a nano-thin layer of polymer, and those materials have been investigated extensively for their mechanical strength. Eventually, the term “aerogel” has been broadened to include “secondary” materials best represented by carbon aerogels, which are obtained from pyrolysis of several sol-gel derived polymeric aerogels (e.g., see: Brinker, C. J., et al., Sol-Gel Science. The Physics and Chemistry of Sol-gel Processing. Academic Press: New York, 1990).
Many aerogels exhibit fragility and are produced by methods that require supercritical fluid (SCF) extraction steps. These shortcomings have hampered commercialization. In one embodiment, the invention disclosed herein overcomes these shortcomings; it describes secondary SiC and Si3N4 aerogels, and metal aerogels, derived from xerogels rather than aerogels, as discussed below.
Organic/inorganic interpenetrating networks include oxide aerogels (e.g., of Cr, Fe, Co, Ni, Cu, Ti, Hf, Sn, and the like) whose skeletal framework is intertwined with a second network of a carbonizable phenolic-resin aerogel (e.g., resorcinol-formaldehyde, polybenzoxazine, and the like). Mimicking the age-old smelting process (e.g., see Leventis, N., et al., J. Mater. Chem., 19:63-65 (2009)), those materials undergo carbothermal reduction, and have been a source for several metallic (e.g., Fe, Co, Ni, Cu) and ceramic (e.g., TiC, Cr3C4, HfC) aerogels (e.g., see Mahadik-Khanolkar, S., et al., Chem. Mater., 26:1318-1331 (2014); Leventis, N., et al., J. Mater. Chem., 20:7456-7471 (2010)). Importantly, it was reported that chemically identical interpenetrating xerogels underwent carbothermal reduction at temperatures that were up to 400° C. lower than those for the corresponding aerogels. Without being bound by theory, this may be taken to indicate that reactions, even amongst nanostructured reagents, may still benefit from a more intimate contact like the one that is found in a more compact structure, i.e., that of a xerogel versus that of an aerogel. Along these lines, it was contemplated herein that the ultimate proximity between an inorganic oxide framework and a carbonizable polymer may be found in nanostructured oxide networks coated conformally with a carbonizable polymer.
As part of an embodiment of the invention herein, a generalizable synthetic protocol that implements the foregoing line of reasoning is illustrated here by the carbothermal synthesis of SiC and Si3N4 aerogels as large shaped-objects using Equations (1) and (2) below, respectively (e.g., see: Saito, M., et al., J. Mater. Sci. Lett., 11:373-376 (1992); Klinger, N., et al., J. Am. Ceram. Soc., 9:369-375 (1966); Bandyopadhyay, S., et al., Ceram. Int., 17:171-179 (1991); Ličlco, T., et al., J. Eur. Ceram. Soc., 9:219-230 (1992); Chung, S. L., et al., J. Mater. Sci., 44:3784-3792 (2009)):SiO2+3C→SiC+2CO  (1)3SiO2+2N2+6C→Si3N4+6CO  (2)The substrate converted to those two ceramics was sol-gel silica coated conformally and cross-linked covalently with carbonizable polyurea from reaction of: (a) innate —OH, and deliberately added —NH2 groups on silica, and (b) adsorbed water, with triisocyanatophenylmethane (TIPM), an available-in-bulk triisocyanate. The process for crosslinking skeletal silica nanoparticles (native or —NH2 modified) with the triisocyanate TIPM is shown in Scheme 1.

Monolithic SiC aerogels have been described before from silica aerogels crosslinked via free-radical surface-initiated polymerization (FR-SIP) of acrylonitrile (see: Leventis, N., et al., Chem. Mater., 22:2790-2803 (2010)). Apart from the inherent synthetic complexity involved with FR-SIP, a main drawback of that approach was also that for porosity it relied on the innate, pre-pyrolysis porosity of the monolithic, crosslinked silica aerogel network. In addition, the topology of the reactants in that arrangement led to mechanically weak materials, and to low utilization of polyacrylonitrile-derived carbon.
In contrast, according to one embodiment of the invention disclosed herein, described below is a TIPM-based methodology that is fast, energy- and materials-efficient, and can be extended to the preparation of other large monolithic ceramic and/or metallic aerogels. In a key aspect, instead of using cross-linked monolithic silica aerogels as the ceramic precursors, the methodology described herein involves preparation and pyrolysis of dry compressed crosslinked silica xerogel powders. These xerogel powders, crosslinked with TIPM-derived polyurea and/or polyurethane coating, have the same nanoparticulate structure as typical monolithic aerogels, but, owing to the short diffusion path in the xerogel powder grains, they can be solvent-exchanged and processed from one step to the next within seconds rather than hours or days. In one aspect, the TIPM-derived polyurea and/or polyurethane coating acts as a binder for the underlying silica particles, so that the dry, crosslinked silica powders can be compressed into large, sturdy compacts with any desirable shape, which effectively removes the autoclave-size limitation from the accessible size of the resulting aerogel articles. And, as importantly, taking isomorphic carbothermal synthesis one step further (e.g., see: Ledoux, M. J., et al., CATTECH, 5:226-246 (2001); Moene, R., et al., Appl. Catal., A, 167:321-330 (1998); Greil, P., 1 Eur. Ceram. Soc., 21:105-118 (2001); Qian, J.-M., et al., J. Eur. Ceram. Soc., 24:3251-3259 (2004); Sonnenburg, K., et al., Phys. Chem. Chem. Phys., 8:3561-3566 (2006)), it was realized that for porosity, polymer crosslinked xerogel powders would rely not on the porosity of the pre-carbothermal object, but rather on the fact that in the course of the carbothermal reduction the carbonizable polymer coating would react away (to the ceramic and CO) (see Equations 1 and 2) creating new porosity that did not exist before. This synthetic design has certain distinct advantages over all prior ceramic aerogel work: First, use of xerogel precursors bypasses supercritical drying, and thus improves energy efficiency. Second, a more subtle feature of working with compressed cross-linked xerogel powders, rather than aerogel monoliths, is that in principle none (or very little) of the reducing agent, CO, which is generated in situ during the course of the reaction, would be carried away; no matter which way from the SiO2/C interface CO wants to move, the compactness of the assembly forces it always through silica, resulting in the most efficient utilization of the carbonizable polymer. Indeed, as disclosed herein, it was just sufficient to work with C: SiO2 ratios near the stoichiometric level, while in the acrylonitrile-crosslinked silica aerogels methodology reported in the literature (see above) that ratio had to be at least 2.5 times higher than the stoichiometric. Eventually, as disclosed herein, pyrolysis of compressed shaped cross-linked xerogel compacts under Ar or N2 yielded same-shape highly porous monolithic SiC or Si3N4, respectively, possessing porosities ≥85%. In contrast, oftentimes in this art porosities up to 30% are considered high. These highly porous ceramic objects of SiC and Si3N4 were mechanically robust, chemically inert at high temperatures, and good thermal insulators. In more general terms, these highly porous SiC and Si3N4 objects are hard ceramics that are useful as abrasives, in cutting tools, and in biomedicine (such as in bone replacement materials). Further, they have industrial usefulness as catalyst supports, or as filters for molten metals, and are prepared by annealing powders under compression. Apart from the immediate relevance of the two model materials disclosed herein to all those industrial applications, the generalizable methodology that is described herewith has brought other porous ceramic and metallic aerogels within its reach, as is disclosed in subsequent embodiments below.
3D Assemblies of polymer-coated silica nanoparticles have been investigated extensively in aerogel form as strong lightweight materials. According to another embodiment of the invention, provided herein is an alternative application for such 3D assemblies of nanoparticles, namely in a novel methodology for carbothermal preparation of sturdy, highly porous SiC and Si3N4 ceramics. This methodology takes into consideration the topology of the carbothermal reactions, and for porosity it relies on the void space created by carbon reacting away. That allows making aerogels from xerogels. Thus, using polymer-crosslinked xerogel powder compacts as the ceramic precursors, rather than monolithic polymer-crosslinked aerogels, processing moves fast, it is energy- and materials-efficient, and most importantly it is generalizable. In that regard, (a) gelation of any system that does so relatively slowly (minutes, hours, or longer) can be significantly expedited by diverting it to powders by vigorous agitation, which is an advantageous key feature of the invention herein; (b) the surface of any sol-gel derived skeletal oxide particle is rich with —OH groups, where isocyanate-derived polymers, like carbonizable TIPM-derived polyurethane and/or polyurea, can latch on covalently; and, (c) crosslinked powders can be compressed to shaped compacts of any size, thus liberating synthesis of ceramic aerogels from the size of the autoclave. In addition to other ceramic aerogels based on refractory materials (e.g., zirconium carbide, and the like; see below), the invention described herein includes Fe(0) metallic aerogels that may alleviate certain issues in thermite applications. Likewise, the invention described herein includes metallic aerogels of Co(0), Ni(0), Cu(0), Ru(0), Au(0), and the like, as described in subsequent embodiments below.
In another embodiment of the invention, disclosed herein are novel, sturdy, highly porous ceramic, metal carbide, metal boride, and metal aerogel monolithic compositions or objects. In a related embodiment, disclosed herein is a method for the synthesis of these sturdy, highly porous ceramic, metal carbide, metal boride, and metal aerogel monolithic compositions or objects from corresponding nanoparticulate polyurea- and/or polyurethane-crosslinked xerogel powder precursors. Said method entails a process that comprises the carbothermal (i.e., pyrolytic) reaction of compressed compacts of the nanoparticulate polyurea- and/or polyurethane-crosslinked xerogel powder precursors. One key aspect of the ceramic and metal aerogel monoliths obtained by this method is that they exhibit high porosity that is ≥35%, even ≥65%, even ≥80%, and even ≥85%. In another aspect, the high porosity in the obtained aerogel monoliths did not exist prior to pyrolysis, but was created via reaction of the core nanoparticles with their carbonized polymer coating toward the new ceramic or metallic framework and the CO that escaped. In another aspect, this method is applicable, and has been demonstrated herein, toward the synthesis of a multiplicity of highly porous ceramic, metal carbide, metal boride, and metal aerogel monoliths, illustratively including, but not limited to, silicon carbide (SiC), silicon nitride (Si3N4), zirconium carbide (ZrC), chromium carbide (Cr3C2), hafnium carbide (HfC), titanium carbide (TiC), zirconium boride (ZrB2), hafnium boride (HfB2), and metallic aerogels of iron (Fe), nickel (Ni), cobalt (Co), copper (Cu), ruthenium (Ru), gold (Au), and others.
Using the compressed compacts of the polymer-crosslinked xerogel powder precursors has several distinct advantages over working with porous monoliths (aerogels or xerogels). First, it accelerates processing, because powders can be washed and solvent-exchanged within seconds rather than hours, due to the short diffusion path. Second, as mentioned above, in compressed compacts, all carbothermal intermediates, especially CO, are forced to go through the reactants, minimizing losses and thus reducing the amount of carbon precursor needed for the conversion to the highly porous ceramic and metal aerogel monoliths, e.g., of silica to SiC or Si3N4. Third, since porosity is created by consuming the carbon precursor, the porous ceramic or metal aerogel is much sturdier than what is obtained if one starts with silica or metal aerogel in porous form.
Thus, in accordance with the above method, highly porous aerogels of SiC and Si3N4 were synthesized as follows. A sol-gel oxide powder (e.g., silica, or a silica precursor; see below) was obtained by disrupting gelation of a silica sol with vigorous agitation. Disrupting gelation via vigorous agitation, accompanied by addition of a solvent (e.g., hexane, and the like), is a key feature of the method herein, because it produces wet-gel powders very rapidly; subsequent processing of the powders (rather than processing monoliths obtained by other methods known in the art) accelerates the whole process tremendously, because the diffusion path in the tiny grains of powder is orders of magnitude smaller than the diffusion path in monoliths; powders can be solvent-exchanged and washed rapidly (e.g., in less than 5 minutes), and can be dried rapidly by simply pulling a vacuum on them. The grains of the obtained powder were about 50 μm in size, irregular in shape, and consisted of 3D assemblies of silica nanoparticles as in any typical silica gel. The individual elementary silica nanoparticles within the grains of the powder were coated conformally with a nano-thin layer of carbonizable polyurea and/or polyurethane derived from the reaction of a polyisocyanate such as an aromatic triisocyanate (e.g., triisocyanatophenylmethane (TIPM)) with the innate —OH groups, deliberately added —NH2 groups, and adsorbed water on the surface of the silica nanoparticles, to yield cross-linked silica powder. The resulting wet-gel powder was solvent-exchanged with a suitable solvent, such as pentane, and the like, and dried at ambient temperature under vacuum. The resulting free-flowing polyurea- and/or polyurethane-coated silica xerogel powder was vibration-settled in suitable dies and was compressed to convenient shapes (e.g., discs, cylinders, donut-like objects, and the like), which in turn were converted to same-shape SiC or Si3N4 artifacts by pyrolysis, e.g., at 1500° C. under Ar or N2, respectively. The overall synthesis was time-, energy-, and materials-efficient. (a) Solvent exchanges within the grains of powder took seconds rather than hours or longer in literature-reported methods; (b) drying did not require high-pressure vessels and supercritical fluids; and, (c) the utilization of the carbonizable polymer was at almost the stoichiometric ratio, due to the xerogel compactness. The final ceramic objects were chemically pure, sturdy, and chemically inert as expected. Pure iron and nickel aerogels (as well as a variety of other metal, metal carbide and metal boride aerogels; see below) were produced via a similar method from sol-gel-derived feria and nickel oxide powders.
The foregoing embodiments of the invention, and additional embodiments, are described in greater detail in the Detailed Description section and the Examples section below.
All publications cited throughout this application are incorporated herein by reference in their entirety. Indeed, throughout this description, including the foregoing description of related art and cited publications, as well as any and all publications cited in what follows below, it is to be understood that any and all publicly available documents described herein, including any and all cited U.S. patents, patent applications, and non-patent publications, are specifically incorporated by reference herein in their entirety. Nonetheless, the related art and publications described herein are not intended in any way as an admission that any of the documents described therein, including pending U.S. patent applications, are prior art to embodiments of the present disclosure. Moreover, the description herein of any disadvantages associated with the described products, methods, and/or apparatus, is not intended to limit the disclosed embodiments. Indeed, embodiments of the present disclosure may include certain features of the described products, methods, and/or apparatus without suffering from their described disadvantages.
Naturally, further objects of the invention are disclosed throughout other areas of the specification, drawings, and claims.